Respiration humidifiers generally used at present (as that described in, e.g., DE 195 08 803 A1) use a humidification chamber, in which heated water is distributed over a large surface. The respiratory gas is passed over this surface. During the contact with the water, the respiratory gas is heated and humidified. This system does not remain sterile, because it is in connection with both the ambient air and the returning water of condensation of the inspiration tube. In addition, this system has too high a compliance, which makes use difficult precisely in the case of premature and full-term newborn babies. The wish to integrate the respiration humidifier within the respirator is hindered by the size of the humidification chamber, as well as its position-dependent function.
The respiration humidifier described in DE 196 21 541 C1 has a membrane type humidifier with a hollow fiber module, which maintains the desired sterility of the water over a long time and also only has a small size. The drawback is that the breathing resistance is not negligible; it is 2 mbar at a respiratory gas flow rate of 60 L/minute. The breathing resistance is especially significant in cases in which the respirator fails and the patient must be supplied spontaneously via an emergency respirator. Excessively high breathing resistances cannot be overcome by the patient. Another drawback of this respiration humidifier is that the hollow fiber module has a wet surface on the respiration side, which may become contaminated with microorganisms after a certain time. These respiration modules must therefore be cleaned and sterilized or completely replaced as disposable parts at regular intervals. This leads to correspondingly high operating costs in the case of this system.
Another possibility of humidifying the respiratory gas is described in DE 43 03 645 C2. A sintered material is placed into a water bath having constant water level and heated. The respiratory gas sweeps past the sintered material, is heated and humidified. This system is intended for humidification in the case of insufflation, while the respiratory gas flow is constant. It is not suitable for respiration, because the humidity and the temperature cannot be controlled independently from one another. The breathing resistance and the compliance are too high. In addition, it is an open system from a hygienic viewpoint, both from the water supply side (with a float chamber, which is in connection with the ambient air), and from the respiratory gas side. The sintered surface may become contaminated very rapidly during periods of no respiration. The operating temperature is even favorable for the formation of microorganisms, and the sintered material with its fine pores is especially accommodating for microorganisms.
Another respiration humidifier has been known from DE-PS 27 02 674; water is boiled off in this humidifier in an evaporation chamber and the respiratory gas saturated with water vapor is sent to a superheater, which is controlled by the respiratory gas temperature of the patient system. The water supply is not separated from the outside air in a sterile manner. The evaporation chamber and the superheater are directly in the respiratory gas system and they must therefore be cleaned and sterilized before they are used for another patient. The design is correspondingly complex. The application of such a system to respirators has not proved successful, either.
Another prior-art respiration humidifier (see DE 43 12 793 C2) uses a heated evaporation chamber, to which water is fed via an injection needle. The evaporation chamber is maintained at a temperature of about 120.degree. C.
A respiration humidifier has been known from DE-AS 25 16 496; this respiration humidifier has the drawback in practice that it is set at a constant evaporation capacity and operated in an uncontrolled manner. As a result, it heats the respiratory gas at different intensities, depending on the existing flow rate. The humidification of the respiratory gas is also uncontrolled; it is obtained from the heating power set and the flow rate. Either the respiratory gas is supersaturated, which correspondingly causes condensation into the condensate container provided for that purpose, or the respiratory gas is humidified insufficiently.
According to a completely different procedure, the water needed for the humidification is metered directly with a pump and is evaporated in a heating chamber (see, e.g., EP 0 716 861 A1, which shows a hose pump and a chamber for evaporating anesthetics). Even though such devices are technically more complicated, because they must actively meter the amount of water in proportion to the respiratory gas flow, they can be made very small, and they do not generate, in general, any additional breathing resistance.
Finally, DE 41 16 512 A1 describes an anesthetic evaporator, in which the respiratory gas flows through a heated, porous sintered material. If the anesthetic evaporator were used as a respiration humidifier, it would heat and humidify the breathing gas. However, separate heating and humidification of the respiratory gas is not possible in this arrangement. In addition, the respiratory gas would come directly into contact with the liquid, which could lead to problems in terms of sterility.
To complement the background information, one should mention the use of passive artificial noses (HME: Heat and Moisture Exchangers), which assume the bridged-over function of the natural upper airways (see DE 41 30 724 A1). These HMEs are adapted by necessity to the Y-piece of the breathing tube system, i.e., to the connection of the tube. The warm and humid air is stored in a moisture and heat exchanger during breathing out by the patient, and it is again released during breathing in. It was possible to markedly improve the efficiencies of such systems in the past years due to improved materials of the exchange surface. As a result, these systems have been increasingly used for the long-term respiration of adults. The technical effort is small. They are, in general, disposable systems, which are removed and replaced with new ones at regular intervals. Yet, the humidification and heating capacity (line) of the systems is insufficient for especially ill patients. There have therefore been developments aimed at improving this passive system by an active humidification and heating (see, e.g., EP 0 567 158 A2); however, this is again technically complicated and leads to the need to lead many cables and tubes to the patient.
These artificial noses also have another serious drawback, which is inherent to the system: The breathing resistance is very high. Another exacerbating factor is that the systems very rapidly become contaminated by the aspiration of the patient and they also become clogged in this case. In many modes of respiration, the clogging of the artificial nose cannot be detected by the monitoring means, so that such systems may bring the patient into a hazardous situation, unless the internal pulmonary pressure (or esophageal pressure) is directly measured. However, being invasive measurements, such measurements are currently not accepted in practice and they also contradict the search for a simple system.